ISA
Shahone. I presume you are not a pilot. What Felixthecat is referring to is International Standard Atmosphere (ISA) which you can find, among other places, on Wikipedia. ISA is 1013.25 hPa pressure and 15 degrees C at Sea Level. As altitude is increased the temperature will usually decrease by approximately 2 degrees C per 1000 feet (up to about 36000 feet) and pressure decreases by roughly 1 hPa per 30 feet. Hope this is helpful.